In the processing of molten aluminum, oxidation forms aluminum dross, chiefly aluminum oxide with varied amounts of impurities. The dross is skimmed from the molten metal and is processed in rotary furnaces with fluxes of sodium and potassium chlorides which are added to promote dross separation and eliminate gas inclusion. The processed dross is discharged as a waste which contains chiefly aluminum oxide, water soluble salts such as sodium and potassium chlorides, substantial quantities of aluminum metal and lesser quantities of impurities such as aluminum carbide and aluminum nitride.
The dross is further processed through aluminum recovery plants which mill and screen the dross and recover the coarse particles which are substantially pure aluminum. The processed aluminum dross (dross tailings) is discharged as an aqueous slurry from which the waste solids are settled and then discharged to a storage area.
The impurities in the dross tailings have, heretofore, precluded utilization of the alumina in the tailings and, consequently, the dross tailings have accumulated over the years in piles adjacent aluminum recovery plants. These piles are unsightly and present environmental problems since the water soluble salts in the tailings are leached by rain and are discharged to surface and ground waters.
Aluminum sulfate has, in recent years, been in increasing demand, principally for use as a floculant in water clarification for industrial and sewage water treatment plants. The aluminum sulfate is currently manufactured by the reaction of sulfuric acid with valuable aluminum sources such as aluminum oxide trihydrate and/or bauxite. Although the aluminum dross tailings would appear to be a less expensive source raw material for aluminum sulfate, the contaminants present in the dross tailings effectively have prevented utilization of this waste material.